With the development of science and technology, microphones, portable computers and other electronic products have achieved ever greater levels of miniaturization. In order to accommodate these products, inter-layer structure of multilayer printed circuit boards has become more and more complicated, thereby leading to the development of high-density interconnection (HDI) technology using via-holes.
Via-holes refer to plated holes that connect conductive traces from one layer to other layers of a multilayer printed circuit board. Via-holes facilitate HDI of multilayer printed circuit boards. Via-holes generally include blind vias, buried vias or through-hole vias. Blind vias are usually used for connecting the conductive traces from an outer layer to an inner layer of a multilayer printed circuit board. Buried vias are usually used for connecting the conductive traces between inner layers of a multilayer printed circuit board. Through-hole vias are usually used for connecting the conductive traces on each layer of a multilayer printed circuit board. In HDI technology, referring to FIG. 3, a typical blind via with stepped structure is shown. Because the blind via is formed by two vias with different diameters stacked together, blind vias like this are generally called stacked via-holes.
Referring to FIGS. 4A-4G, as an example, a method for forming stacked via-holes in multilayer printed circuit boards generally includes the steps of: firstly, referring to FIG. 4A, applying a substrate 100, which includes a base film 101 and two conductive layers 102 onto two opposite surfaces of the base film 101 thus forming conductive traces; secondly, referring to FIG. 4B, attaching a first film 110 onto each of the two conductive layers 102, each first film 110 including a first resin layer 114 and a first conductive layer 112 thereon, the first resin layer contacting with the conductive layer 102; thirdly, referring to FIG. 4C, forming a number of first windows 130 at predetermined positions of the first conductive layer 112 using a photolithographic process; fourthly, referring to FIG. 4D, attaching a second film 120 on each of the two first films 110, each second film 120 including a second resin layer 124 and a second conductive layer 122 thereon, the second resin layer 124 contacting with the first conductive layer 112; fifthly, referring to FIG. 4E, forming a number of second windows 140 with larger diameters in different positions of the second conductive layer 122 corresponding to the first windows 130 using a photolithographic process; sixthly, referring to FIG. 4F, removing the second resin layer 124 corresponding to the second windows 140 and the first resin layer 114 corresponding to the first windows 130 using a carbon dioxide laser to form a number of stacked via-holes 150; finally, referring to FIG. 4G, plating the stacked via-holes 150 to connect the conductive traces.
However, the typical method for forming stacked via-holes 150 in the multilayer printed circuit board described above has the following disadvantages. First of all, because formation of the second windows 140 is based on formation of the first windows 130, the first windows 130 in the first film 110 and the second windows 140 in the second film 120 can undergo positional excursion, thereby affecting precision of the stacked via-holes 150. Secondly, during formation of the first windows 130 and the second windows 140, the photolithographic process used many times can suffer from increasing system error. Thirdly, the laser treatment is used only once, simultaneously removing both the first resin layer 124 corresponding to the first window 130 and the second resin layer 124 corresponding to the second windows 140, so the strength of the laser cannot be adjusted according to windows with different diameters and resins with different characteristics. Referring to FIGS. 5A and 5B, different views of a typical poorly-made stacked via-hole formed due to above-described reasons are shown. The poorly-made stacked via-hole can cause potential short circuits, thereby affecting quality of final products.
What is needed, therefore, is a method for forming stacked via-holes in multilayer printed circuit boards to improve quality of the stacked via-holes.